1. Field of the Invention
The present invention relates to a method and a related apparatus for driving an LCD monitor, and more particularly, to a method and a related apparatus of applying class-A operational amplifiers for driving the LCD monitor.
2. Description of the Prior Art
The advantages of a liquid crystal display (LCD) include lighter weight, less electrical consumption, and less radiation contamination. Thus, the LCD monitors have been widely applied to several portable information products, such as notebooks, PDAs, etc. The LCD monitors gradually replace the CRT monitors of the conventional desktop computers. Incident light will produce different polarization or refraction effects when the alignment of liquid crystal molecules is altered. The transmission of the incident light is affected by the liquid crystal molecules, and magnitude of the light emitting out of liquid crystal molecules varies. The LCD monitor utilizes the characteristics of the liquid crystal molecules to control the corresponding light transmittance and produces gorgeous images according to different magnitude of red, blue, and green light.
Please refer to FIG. 1, which is a diagram of a prior art thin film transistor (TFT) LCD monitor 10. The LCD monitor 10 has an LCD panel 12, a controller 14, a first driving circuit 16, a second driving circuit 18, a first voltage generator 20, and a second voltage generator 22. The LCD panel 12 is constructed by two parallel substrates. There is an LCD layer stuffed into space between these two substrates. A plurality of data lines 24, a plurality of gate lines 26 that are perpendicular to the data lines 24, and a plurality of thin film transistors 28 are positioned on one of the substrates. There is a common electrode installed on another substrate, and the first voltage generator 20 is electrically connected to the common electrode for outputting a common voltage Vcom via the common electrode. Please note that only four thin film transistors 28 are shown in FIG. 1 for clarity. Actually, the LCD panel 12 has one thin film transistor 28 installed in each intersection of the data lines 24 and gate lines 26. In other words, the thin film transistors 28 are arranged in a matrix format on the LCD panel 12. The data lines 24 correspond to different columns, and the gate lines 26 correspond to different rows. The LCD monitor 10 uses a specific column and a specific row to locate the associated thin film transistor 28 that corresponds to a pixel. In addition, the structure of the LCD panel 12, that is, two substrates with one LCD layer is equivalent to a capacitor 30. The substrates function as conductive plates, and the stuffed LCD layer functions as a dielectric.
The operation of the prior art LCD monitor 10 is described as follows. When the controller 14 receives a horizontal synchronization signal 32 and a vertical synchronization signal 34, the controller 14 generates corresponding control signals respectively inputted into the first driving circuit 16 and the second driving circuit 18. The first driving circuit 16 and the second driving circuit 18 then generate input signals to the LCD panel 12 for turning on the corresponding thin film transistors 28 so that a voltage difference will be kept by the capacitors 30. For example, the second driving circuit 18 outputs a pulse to the gate line 26 for turning on the thin film transistor 28. Therefore, the voltage of the input signal generated by the first driving circuit 16 is inputted into the capacitor 30 through the data line 24 and the thin film transistor 28. The voltage difference kept by the capacitor 30 can further adjust a corresponding gray level of the related pixel through affecting the related alignment of liquid crystal molecules positioned inside the LCD layer. In addition, the first circuit 16 generates the input signals, and magnitude of each input signal inputted to the data line 24 is controlled by the second voltage generator 22. Different voltage levels generated by the second voltage generator 22, therefore, correspond to different gray levels.
If the LCD monitor 10 continuously uses a positive voltage to drive the liquid crystal molecules, the liquid crystal molecules will not quickly change a corresponding alignment according to the applied voltages as before. Thus, the incident light will not produce accurate polarization or refraction, and the quality of images displayed on LCD monitor 10 deteriorates. Similarly, if the LCD monitor 10 continuously uses a negative voltage to drive the liquid crystal molecules, the liquid crystal molecules will not quickly change a corresponding alignment according to the applied voltages as before. Thus, the incident light will not produce accurate polarization or refraction, and the quality of images displayed on the LCD monitor 10 deteriorates. In order to protect the liquid crystal molecules from being irregular, the LCD monitor 10 must alternately use the positive and the negative voltage to drive the liquid crystal molecules. In addition, not only does the LCD panel 12 have the capacitors 30, but also the related circuit will have some parasite capacitors owing to its intrinsic structure. When the same image is displayed on the LCD panel 12 for a long time, the parasite capacitors will be charged to generate a residual image effect. The residual image with regard to the parasite capacitors will further distort next images displayed on the same LCD panel 12. Therefore, the LCD monitor 10 must alternately use the positive and the negative voltage to drive the liquid crystal molecules for eliminating the bothering residual image effect. However, when the LCD monitor 10 alternately uses the positive and negative voltage to drive the liquid crystal molecules, the image displayed will flicker owing to a voltage offset generated by the thin film transistor 28. The reason is described as follows.
Please refer to FIG. 2, which is an output voltage diagram of the second voltage generator 22 shown in FIG. 1. As with the voltages V0, V1, V2, V3, V4, V5, V6, V7, V8, V9 shown in FIG. 2, the second voltage generator 22 generates different voltages according to display data 36 for driving the thin film transistors 28 positioned on the LCD panel 12. However, when the thin film transistor 28 is turned on, the voltage difference between the input terminal and the output terminal of the thin film transistor 28 is equal to a deviation Vd. Therefore, the actual values of voltages such as V20, V21, V22, V23, V24, V25, V26, V27, V28, V29 imposed on the LCD panel 12 are individually lower than the corresponding ideal values of voltages such as V0, V1, V2, V3, V4, V5, V6, V7, V8, V9. As mentioned above, the LCD monitor 10 alternatively uses the positive and negative voltages to drive each pixel on the LCD panel 12. In other words, the voltage outputted from the second voltage generator 22 is changed so that the voltage difference between the voltage outputted from the second voltage generator 22 and the common voltage Vcom generated by the first voltage generator 20 has an alternating polarity. For example, the display data 36 indicates that a voltage difference V1−Vcom is required to drive one pixel, and the pixel will hold the voltage difference V1−Vcom during a predetermined interval. Because the pixel is alternatively driven with the positive and negative voltages, the positive voltage V1−Vcom and the negative voltage (Vcom−V8) are alternatively imposed on the LCD panel 12. However, the actual voltage V21−Vcom is not equal to the voltage Vcom−V28 owing to the deviation Vd of the thin film transistor 28. Therefore, when the pixel is alternatively driven with the positive voltage V21−Vcom and the negative voltage (Vcom−V28), the pixel flickers because of an unstable gray level.
Please refer from FIG. 3A to FIG. 6B. FIG. 3A and FIG. 3B are diagrams of a prior art line inversion driving method. FIG. 4A and FIG. 4B are diagrams of a prior art column inversion driving method. FIG. 5A and FIG. 5B are diagrams of a prior art dot inversion driving method. FIG. 6A and FIG. 6B are diagrams of a prior art two-dot line inversion driving method. In order to solve the mentioned problem when the LCD monitor 10 alternatively uses the positive and negative voltages to driving the liquid crystal molecules, the LCD monitor 10 adopts the line inversion driving method, the column inversion driving method, the dot inversion driving method, or the two-dot line inversion driving method to eliminate the image flickers. In the drawings from FIG. 3A to FIG. 6B, a first image frame 42 and a second image frame 44 are two successive image frames. The polarity of the pixel 46 in the first image frame 42 is opposite to the polarity of the same pixel 46 in the second image frame 44. In addition, the line inversion driving method, the column inversion driving method, the dot inversion driving method, and the two-dot line inversion driving method are classified according to different polarity arrangements of pixels 46. As shown in FIG. 3A to FIG. 6B, the line inversion driving method can eliminate image flickers along the vertical direction, and the column inversion driving method can eliminate image flickers along the horizontal direction. Similarly, the dot inversion driving method and the two-dot line inversion driving method both can eliminate image flickers along the vertical direction and the horizontal direction simultaneously for improving corresponding image quality.
Please refer to FIG. 5A, FIG. 5B, and FIG. 7. FIG. 7 is a voltage diagram of the dot inversion driving method shown in FIG. 5A and FIG. 5B. For the pixels 47 positioned in different rows 50, 52, 54, 56 but the same column 48, the polarities of the pixels 47 in the first image frame 42 are “positive”, “negative”, “positive”, and “negative” respectively. Therefore, a class-A operational amplifier buffer is used for pulling up voltages of the pixels 47 in rows 50, 54 so that the pixels 47 will have the same positive polarity. Similarly, another class-A operational amplifier buffer is used for pushing down voltages of the pixels 47 in rows 52, 56 so that the pixels 47 will have the same negative polarity. In other words, the prior art dot inversion driving method must use one class-A operational amplifier buffer for driving pixels with positive polarities, and another class-A operational amplifier buffer for driving pixels with negative polarities.
Please refer to FIG. 6A, FIG. 6B, and FIG. 8. FIG. 8 is a voltage diagram of the two-dot line inversion driving method shown in FIG. 6A and FIG. 6B. For the pixels 47 positioned in different rows 50, 52, 54, 56 but the same column 48, the polarities of the pixels 47 in rows 50, 52, 54, 56 within the first image frame 42 are “positive”, “positive”, “negative”, and “negative” respectively. Therefore, a class-AB operational amplifier buffer must be used for pulling up and pushing down voltages of each pixel 47 in rows 50, 52 so that the pixels 47 will have the same positive polarity. Similarly, another class-AB operational amplifier buffer must be used for pushing down and pulling up voltages of each pixel 47 in rows 54, 56 so that the pixels 47 will have the same negative polarity. In other words, the prior art two-dot line inversion driving method must use one class-AB operational amplifier buffer for driving two successive pixels with the same positive polarity, and another class-AB operational amplifier buffer for driving two successive pixels with the same negative polarity.
When the dot inversion driving method encounters a flicker pattern, the two-dot line inversion driving method is used for handling the flicker pattern to reduce the related flicker problem. Please refer to FIG. 5A, FIG. 5B, FIG. 6A, and FIG. 6B. As shown in FIG. 5A and FIG. 5B, the pixels 47 are divided into two groups, that is, the pixels 47 with circle symbols, and the pixels 47 without circle symbols. Concerning the flicker pattern, the pixels 47 with circle symbols have an identical gray level equivalent to 0 volts, and the polarity of each pixel 47 with a circle symbol will not change in both image frames 42, 44 according to the dot inversion driving method. On the contrary, all of the pixels 47 without circle symbols will change polarities from “negative” to “positive” according to the dot inversion driving method. However, only the pixels without circle symbols will change polarities in different image frames 42, 44, and the pixels without circle symbols have the same polarity in each frame 42, 44. Therefore, the flicker pattern with regard to the above-mentioned dot inversion driving method is equivalent to pixels driven according to a prior art frame inversion driving method. In other words, all of the pixels 47 without circle symbols in the first image frame 42 have the same negative polarity, and all of the pixels 47 without circle symbols in the second image frame 42 have the same positive polarity. Therefore, a flicker problem generated owing to this display pattern. Therefore, the two-dot line inversion driving method is adopted for solving the flicker problem mentioned above. Please refer to FIG. 6A and FIG. 6B. The pixels 47 with circle symbols will not change polarities in the first and second image frames 42, 44, but the pixels 47 without circle symbols will. For the pixels 47 without circle symbols shown in FIG. 6A, each pixel 47 has its specific polarity such as a positive polarity or a negative polarity. It is the same for the pixels 47 without circle symbols shown in FIG. 6B, and the pixels 47 without circle symbols will generate “mixed” polarity transitions among adjacent pixels 47 between the first and second image frames 42, 44 to reduce the flicker problem mentioned above.
As mentioned above, with regard to the dot inversion driving method, the prior art LCD monitor 10 adopts the class-A operational amplifier buffer to drive pixels. However, in the two-dot line inversion, the prior art LCD monitor 10 has to adopt the class-AB operational amplifier buffer to drive pixels. Though the flicker pattern does not occur frequently, the prior art LCD monitor 10 still requires the class-AB operational amplifier buffer to handle the possible flicker pattern. Therefore, the prior art LCD monitor 10 can not only use class-A operational amplifiers buffer to drive pixels according to the dot inversion driving method and the two-dot line inversion driving method.